This application claims foreign priority benefits of German Patent Application No. 10110658.0 filed Mar. 6, 2001.
1. Field of the Invention
The present invention relates generally to vehicle brake systems and specifically to a pumping apparatus for a vehicle brake system, which is capable of compensating pressure differences in brake circuits and different pumping capacities for brake circuits.
2. Background of the Invention
In vehicle brake systems, which comprise e.g. an antilock braking system (ABS), electronic/hydraulic brake assist (EBA/HBA), a traction control system (TCS), an electronic stability program (ESP), vehicle stability control (VSC) and the like and/or are operated in conjunction with automatic cruise control (ACC), pumps associated with individual brake circuits are usually used in order in the event of a pressure reduction for reducing braking forces to pump fluid, which has flowed off from wheel brakes, back into the appropriate brake circuit.
Differing pumping capacities may give rise to differences between the brake pressures in the brake circuits. This may lead not only to an unwanted reduction of the vehicle deceleration produced by means of wheel brakes but also to unwanted or non-controllable driving states (e.g. xe2x80x9cbreaking awayxe2x80x9d or skidding of the vehicle). Especially in vehicle brake systems with a diagonal split, this may lead to undesirable or non-controllable yaw moments.
Thus, e.g. for a vehicle brake system having two brake circuits and an antilock braking system, radial piston pumps are used. The pumps are driven by means of an eccentric, which is seated on a shaft driven by an electric motor. In order to pump brake fluid back into the brake circuits, fluid accommodated in pressure chambers of the pumps is fed back by reducing the volume of the pressure chambers. Differing pumping capacities of the pumps may lead to the problems described above. Furthermore, the piston movements needed for operation are, particularly because of the eccentric drive, not linear. As a result, for example, pulsations of a brake pedal during operation of the antilock braking system are influenced in a manner which is undesirable and also perceptible to a motor vehicle driver.
From U.S. Pat. No. 4,962,972 it is known in a pumping apparatus for a vehicle brake system to dispose bushes, which may move in dependence upon piston movements, movably in a housing. Pistons, which are movably disposed in the bushes, are moreover provided.
An object of the present invention is to provide an apparatus for compensating the pressure in brake circuits of a vehicle brake system. In particular, the present invention is to enable pressure compensation between brake circuits of a controlled vehicle brake system when, during operation of the latter, brake fluid is fed back into the brake circuits.
The fundamental approach of the present invention is to dispose bushes, in which pistons of a pumping apparatus for a vehicle brake system are movably disposed, in such a movable manner that they themselves, in dependence upon movements of the pistons, act as pistons in order to supply brake fluid to the vehicle brake system. In particular, the bushes are designed to be movable in such a way that, in response to piston movements, they deliver brake fluid into a brake circuit of the vehicle brake system, in which no brake fluid is displaced as a result of a pumping operation caused by the pistons themselves. This means that bushes, whose pistons are delivering brake fluid into an associated brake circuit, do not act as pistons, while bushes, whose pistons are delivering substantially no brake fluid into an associated brake circuit but are e.g. taking in brake fluid removed from the vehicle brake system in order subsequently to pump it back into the brake circuit, are moved in such a way that they develop a pumping action for said brake circuit.
In said manner brake fluid is supplied to brake circuits, into which no brake fluid is pumped by their associated pistons. As a result, during each pumping operation caused by a piston brake fluid is pumped into all of the brake circuits connected to the pumping apparatus according to the invention, and indeed is pumped irrespective of whether the associated pistons are pumping back brake fluid or not, because in the case of the latter the appropriate bushes act as pistons.
In order during a pumping operation caused by pistons to supply substantially the same quantity of brake fluid to each brake circuit and hence avoid pressure differences between the brake circuits, the bushes in dependence upon piston movements, which deliver brake fluid back into brake circuits, are moved in such a way that the brake fluid volumes displaced by the bushes correspond to the volumes, which are pumped back by the pumping pistons.
If different pressures are to exist in brake circuits of a vehicle brake system, the bushes are to be designed in such a way that their movements in response to movements of pumping pistons result in a displacement of greater and/or smaller brake fluid volumes.
The present invention is also based on the realization that (unwanted) pressure differences in brake circuits of a vehicle brake system caused by different pumping capacities of associated pumping devices may be compensated by throttling the supply of brake fluid to pumping devices of a higher pumping capacity. According to the invention such throttling is effected by movements of the bushes, thereby avoiding additional devices and/or measures for detecting pressures in brake circuits and pumping capacities as well as for throttling the supply of brake fluid.
The present invention specifically provides a pumping apparatus for a vehicle hydraulic or pneumatic brake system, which apparatus comprises a housing and pistons movably disposed therein. The pistons are movably disposed in each case in bushes, which in turn are disposed in the housing so as to be movable in dependence upon piston movements.
According to the invention the bushes cooperate in such a way that movements of a bush, whose piston is moved in the sense of an operation whereby brake fluid is pumped into a brake circuit of the vehicle brake system, give rise to movements of the other bush(es).
Said active connection between the bushes may be achieved by mechanically connecting the bushes or integrating the latter into a constructional unit. It is further provided to use actuating members, which are disposed on the bushes and which in dependence upon movements of the bushes may establish active connections between the latter in order to transmit forces for movements of the bushes.
The cooperating bushes preferably form a plunger piston, which acts in the housing.
In particular, the bushes with the pistons disposed therein each delimit a pressure chamber, wherein each bush forms an outlet chamber in the housing. In dependence upon piston movements fluid connections between the pressure chambers and associated outlet chambers are established and interrupted. For said purpose non-return valves may be used, which open or close the outlet openings of the pressure chambers.
To enable brake fluid to be supplied to brake circuits of the vehicle brake system, which are in communication with an outlet chamber, of which the fluid connection to the corresponding pressure chamber has been interrupted, the plunger piston and, in particular, the bush forming such an outlet chamber is disposed in such a movable manner that it acts in said outlet chamber as a piston and pumps brake fluid into the associated brake circuit.
Preferably, elastic restoring elements such as e.g. restoring springs for the bushes are used, which counteract forces of motion caused by piston movements. By said means inoperative positions for the bushes and hence an inoperative position for the piston plunger are defined.
In one embodiment, the housing comprises two movably disposed pistons, which develop their pumping action for an associated brake circuit with a phase displacement of 180xc2x0. Accordingly, two bushes are used there, wherein the bush, whose piston is not moved in the sense of a pumping operation, acts in the housing as a piston in order to deliver brake fluid into the associated brake circuit.
In a further embodiment, the pumping apparatus comprises three bushes, which are movably disposed in the housing and in each of which a piston is movably disposed, wherein the pistons develop their pumping action for associated brake circuits with a phase displacement of 120xc2x0. In said embodiment the two bushes, whose pistons are not delivering brake fluid back into associated brake circuits, pump brake fluid back to the vehicle brake system, while the brake circuit fluidically connected to the pressure chamber of the pumping piston is supplied by the latter.
It is further provided that there is associated with each bush a throttling device, which in dependence upon movements of the pistons and of the corresponding bush limits the supply of brake fluid.
For said purpose the bushes may have supply bores, which are fluidically connected, e.g. via inlet chambers, to corresponding inlet openings. In dependence upon movements of the bushes relative to the housing movements of the supply bores relative to the inlet bores arise, with the result that a throttling of the supply of brake fluid may be caused.
The supply bores and the inlet bores are preferably disposed relative to one another in such a way that the supply of brake fluid occurs in dependence upon bush movements. Thus, for example, it is provided that a significant throttling of the supply of brake fluid occurs only when the corresponding bush has moved far enough.
It is further to be preferred that the supply bores and the inlet bores are disposed and/or the bushes are movable in such a way that the supply of brake fluid is throttled for the piston, which compared to the other piston(s) has a higher pumping capacity.
A further advantage arises when the supply bores and the inlet bores are movable relative to one another in such a way that the supply of brake fluid is improved for the piston(s), which compared to the other pistons has a lower pumping capacity. In combination with the throttling action for the piston of a higher pumping capacity, this leads to a faster compensation of (unwanted) differences between the brake fluid volumes delivered back into the brake circuits.